Pope Benedict XVI's resignation came like a bolt from the blue overnight.

And the weather around the Vatican was eerily appropriate, with lightning striking St Peter's Basilica, one of the holiest Catholic sites, on the same day that Pope Benedict announced he would be stepping down.

Global news agency Agence France-Presse published an image of lightning striking the basilica's dome, which it said was taken "on the day the Pope" announced his resignation.

AFP said the striking image was captured by photographer Filippo Monteforte, who works for Italian national news and photo agency ANSA.

Monteforte's website shows that he has photographed the Pope extensively for for more than a decade. He is also listed as a AFP photographer, with a portfolio of his work on the news agency's website.

The image was doing the rounds on social media overnight, with some people questioning its authenticity.

Fairfax Media photographer Nick Moir said the image looked genuine.

"It's probably not that rare for St Peter's to get hit," he said.

"The bolt is hitting a lightning rod to the side of the cross, it seems."

Although Moir said there was no way to verify when the photograph was taken.

Pope Benedict's resignation is the first by a pontiff for almost 600 years, and his decision stunned the Roman Catholic church.

In an address read out in Latin letter to his Catholic brethren,the 85-year-old said his advanced age and the pace of change in the modern world had left him unable to “adequately fulfil the ministry entrusted to me”.

He will step down as head of the Catholic church on February 28.

The Vatican newspaper, L'Osservatore Romano, said he had made up his mind nearly a year ago after trips to Mexico and Cuba tired him out.

His 89-year-old brother, Georg Ratzinger, said: "Age is weighing on him. My brother would like more rest at this age."

Once he stands down, Benedict will be taken to Castel Gandolfo, the papal summer retreat near Rome, and will subsequently live in a cloistered monastery.

Previous Popes have stayed in the role until their death, despite physical and mental decay, in the belief that their prayer and suffering as they approach the end are a part of their role.

The last pope to resign was Pope Gregory XII in 1415, to help resolve a dispute between the three people who claimed to be the Pope.